In normal mode I can console kill anyone, in ironman mode I don't know when the seed is bad. So under what circumstances does this "wait a few days to change the seed" advice work?
There is no advice, and there is no "fix", because nothing is wrong. You are not supposed to know the random outcome before you press the button, and therefore whatever outcome you experience is essentially random for you. Making a single attempt is always correct and random. Live with the result you got.
Waiting for the seed to change is only going to be helpful if you're attempting to do something multiple times - either through save-scumming or legitimately doing an action multiple times in a row - and have noticed that you failed with a high probability of success.
Everything people have been telling you in this thread is that your initial complaint is wrong - the percentages work just fine, and the reason why 100% still fails is because of rounding. I honestly don't know what you're trying to do by trying to find any little tiny thing to argue about, but I'd strongly suggest that if it's just so you don't have to feel like you "lost" this argument that you should just stop. You're only making yourself look foolish.
The imprison chance indicator lies. If you ever go on massive imprisoning sprees you'll see the actual chance is much lower than the one shown. You can definitely tell with all the failed 100% ones, as they happen far more often than 1 in 200 (assuming misleading rounding). I also know this was something changed with patches, as it used to be that 100% meant 100%.
Confirmation bias is a thing. I'm also highly skeptical that you've been imprisoning enough people to get a big enough sample size to be able to test this, especially because the only people I've ever seen with a 100% imprisonment chance are unlanded courtiers - and who bothers to go on a mass imprisonment spree against them?
It's also likely that the mass imprisonment spree is done while the game is paused, which brings up the obvious fact that we've been talking about this entire thread, namely, that the random seed will not have been reset between attempts. This would mean that if an unlikely outcome happens, it's going to keep happening until the seed is reset. Of course you're going to see a greater number of events failing as soon as you get your first failure at 100%, because the seed is one that is going to make those events fail.
You also don't know that 100% previously meant exactly 100%, because, well, if there's rounding that's still an almost assured chance of success. Maybe you just weren't unlucky enough to get a failure at 100% until this patch.
You've no idea what a pleasant surprise is to find a post that makes sense after two pages of bs.
But they're still wrong, or at least don't have good reason to believe that there's anything wrong with the percentages. Maybe you should be approaching this thread by listening to what people are saying, rather than trying to "win" an argument and only listening to people that already agree with you.